Verse translation:
(Lord Viṣṇu showed himself once to Nārada Muni and then told him:)
"O sinless Nārada! I showed myself once to produce a desire in you to see Me. The devotee so desiring Me gradually becomes freed of all material desires."

Commentary:
(Nārada said:) "But just show yourself once more to me ! "
(Lord Viṣṇu replied:) "Seeing Me only once, not many times, is enough to produce desire for Me. By only slight increase in longing, prema will not develop to the state of youthfulness in a person having somewhat weak prema. My rule is that I show myself one time only to a person practicing in his present body (in his sādhaka-deha) who has developed prema. The infant state of prema in the sādhaka's body matures to a youthful state in the siddha-deha by an increase of prema arising from longing in separation. (There seems to be growth still in the siddha deha too, unlike with the impersonalists, whose perfection is static). That youthful prema allows the devotee to see Me constantly and serve Me directly. I alone, and not My devotee, know the process of fulfilling the desires of My devotee. You, who simply desire Me, even without having seen Me, will become free from all desires for material enjoyment (hṛcchayan)."

This statement does not actually apply to Nārada since he did not have any material desires, being at the level of prema already. But saying this, the Lord shows the nature of devotion. Additionally by saying this, the Lord increases the humility of Nārada."